The easiest way to spot a supermassive black hole (SMBH) is when it expels a huge jet of matter in one of the most energetic displays in the Universe. While astronomers have spotted these huge black holes at the centers of most galaxies, not all are active—meaning the jet isn't there, and the SMBH is hiding. However, even inactive black holes may give themselves away if we can spot them eating stars: the disruption of a star by gravitational forces can produce a burst of light.

As reported in Nature, the Pan-STARRS1 (PS1) galaxy survey spotted a burst of intense ultraviolet and visible light from the center of a galaxy with no known SMBH. S. Gezari et al. performed a spectral analysis on the flare, and determined it to be consistent with the destruction of a red giant star with a helium-rich core. The likely culprit for the star's disruption is a black hole with a mass between 2.7 and 2.9 million times that of our Sun.

Gravity will tear us apart

Astronomers have observed SMBHs at the cores of a substantial fraction of large galaxies (including the Milky Way), leading to a widespread consensus that all galaxies have them. In many known SMBHs, a disc of hot gas surrounds the black hole; the acceleration of the gas creates huge jets of particles and produces light in radio and X-rays. These SMBHs are known as active galactic nuclei.

Not every SMBH is active. Some, including our Milky Way, are relatively quiet, requiring other methods to observe and characterize them.

Although black holes can devour objects directly, a more typical scenario involves disruption via the tidal force of gravity. (This is simply a stronger version of the force that raises tides on Earth and makes the same side of the Moon face Earth.) Supermassive black holes exhibit stronger tidal forces, strong enough to tear stars apart—but this phenomenon is rare and transient, so only three such events have ever been spotted, including the present study. From a statistical analysis, astronomers expect one tidal disruption event per galaxy every ten thousand years; watching for them requires either a lot of patience, or a lot of galaxies.

Tidal disruption by SMBH is a function of the black hole's mass as well as the size and mass of the star being torn apart. Stars with large diameters are more likely to be shredded than smaller stars, and lower mass increases the probability as well. (If the black hole is too massive compared to the star, it will swallow the star whole.) During disruption, roughly half the star's mass is swallowed by the black hole, while the remainder is shot out into space at high speed. Unlike gas falling into a black hole, tidal disruption of a star produces visible and ultraviolet light, but not X-rays.

Watch what you eat

While the theory of tidal disruption by SMBHs is well understood, it's another thing to spot it in action. Gezari et al. spotted a bright visible light flare in data from the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS1) survey of galaxies. The flash also appeared in data from the Galaxy Evolution Explorer (GALEX), which is an ultraviolet instrument. Tracking its visibility over time, the astronomers determined the flare was located right at the center of a galaxy, to a high degree of confidence.

The researchers followed up observations using the Chandra X-ray Observatory, and failed to find anything at the location of the flare. This rules out gas accretion onto a black hole, meaning the SMBH candidate is not active. Similarly, the strong ultraviolet signal rules out the possibility that the flare was due to a supernova explosion. On the other hand, the flare showed a high fraction of ionized helium—also something unlikely to be present either in ordinary gas accretion or a supernova.

Matching the spectrum and duration of the flare to the model for tidal disruption, Gezari et al. determined the most likely candidate for the event was a red giant torn apart by a SMBH. Red giants are stars near the end of their lives: they have converted the hydrogen in their cores into helium.

When the astronomers fit both the parameters for the star and the black hole to the data, they determined the star was more massive than our Sun, but not enormously so. They also found a best-fit value for the SMBH mass to be 2.8±0.1 million times the mass of the Sun. (For comparison, the Milky Way's SMBH is about 4 million times the mass of the Sun.)

The lack of gas accretion may rule out the possibility that the SMBH is an active galactic nucleus, but it's also puzzling for tidal disruption. The envelope of the disrupted star should have formed a disc of gas around the black hole, producing accretion behavior that we should be able to observe. The researchers suggest the star may already have lost its outer layers through tidal stripping before the final disruption, or perhaps the envelope may have been blown away through another process, leaving only the helium-rich core.

With large surveys of galaxies to work with, astronomers should be able to spot many more events like this. Observations will settle how often these disruptions occur, and provide a better comparison with the theory, teaching us much about black hole behavior in galaxies.

Wonder what it would be like, to be on a planet orbiting a star where this was about to happen... What would one see, in the "distance"?

By the time the star is close enough to be swallowed by the black hole, chances are there is not a single thing left orbiting said star, as they would have been in the path of the black hole before the star was.

But articles like this make me wish I was born a few hundred years in the future so that we could actually see things like this in person.

Wonder what it would be like, to be on a planet orbiting a star where this was about to happen... What would one see, in the "distance"?

I can't claim to have a serious understanding of the material, but I'd expect that any objects in that system would at the very least have been rendered uninhabitable due to orbital changes long before the star actually started being torn apart...assuming they hadn't already been torn apart or consumed themselves.

In order to get this level of confirmation, we needed at least these three levels of sky-watching; visible light, UV light, and X-ray light. Each of these was managed by a different project with different sets of equipment tuned to look at the same sky in different parts of the spectrum. Some were ground-based and others are orbiting observatories. With so many missions going on to sniff out the secrets of the Universe it really is a great time to be both alive and curious.

From a statistical analysis, astronomers expect one tidal disruption event per galaxy every ten thousand years; watching for them requires either a lot of patience, or a lot of galaxies.

So, I wondered to myself: how many such events would occur on an average day?

Wolfram Alpha told me that the number of galaxies in the observable universe divided by the number of days in 10,000 sidereal years = approx 47,000 events per day, which equates to roughly 33 events per minute.

I guess the trick will be to be looking for the events at the right part of the universe at the right time.

A human might be able to survive the tidal forces of a supermassive black hole, but certainly not planets or star systems.

You'd be turned into oatmeal long before you even realized you were entering the tidal force of a SMBH.

You'd have a better chance of realising it in the case of an SMBH in comparison to a less massive black hole, as the tidal forces near an SMBH are much weaker.

Why?

Yes, in reality, you'd be killed by radiation and probably an intense magnetic field.

But tidal forces go like 1/R^3 (like the derivative of the roughly 1/R^2 force), and the Schwarzschild radius goes like the mass. So, while a supermassive black hole has a larger mass than a stellar black hole, this effect is only linear, so the tidal forces at the event horizon are weaker by a factor of 1/M^2, where M is measured in units of the smaller black hole's mass.

Wonder what it would be like, to be on a planet orbiting a star where this was about to happen... What would one see, in the "distance"?

By the time the star is close enough to be swallowed by the black hole, chances are there is not a single thing left orbiting said star, as they would have been in the path of the black hole before the star was.

But articles like this make me wish I was born a few hundred years in the future so that we could actually see things like this in person.

Hate to bust your bubbles but in few thousands years humanity will all be extinct. Anyway I would love to see whole cosmos up close with my naked eye.imagine the view to look at supernova from your spaceship.

"Wonder what it would be like, to be on a planet orbiting a star where this was about to happen... What would one see, in the "distance"?"

Your planet would airless and hot because the red giant grew out and blasted your previously inhabited world. The huge red disk would start getting tugged at one side and streamers would begin to spiral around something that makes the other stars in the sky twinkle and look weird when they go behind it. If you're lucky the accretion disk would be perpendicular to you so that you don't get blasted by the collimated particle wind coming from the axes.

Then the BH pulls off the red outer part of the star and you see the helium core that looks like a white dwarf. It might get cooler for a bit around your planet because even though much hotter, the core is much smaller. But when the BH takes a bite, your planet and you get vaporized in one big blast.

Thousands of years of fusion energy was working it's way out of the core in a kind of pachinko game where the photons are the balls and the ionized helium were the pins. But the BH ripping apart the core releases the energy all at once, making the light pulse discussed in the article.

Is this impossible, unlikely, or unobserved as yet? IANAS but would have thought that if they're on the correct (i.e. collision) course then they could easily hit each other. Is there something to do with the stars themselves which prevents them colliding and effectively forces them into a binary system?

So, as a black hole gains more mass, it gets less dense based on the second paragraph. Would it then be possible for a black hole to acquire so much mass that it stops being a black hole?

No, what that's saying is that the volume of the region light cannot escape scales as the radius cubed (duh) while mass scales directly with the radius (a property of black holes), such that the "density" goes down as the light-can't-escape region gets larger because an increase in mass by 2 increases the volume of the light-can't-escape region by 8, decreasing the density by a factor of 1/4. Therefore, a 10 million solar mass black hole is 1/(1,000,000)^2 times as "dense" as a 10 solar mass black hole, but a black hole it remains.

I keep putting dense in quotes because in reality every black hole has all of its mass concentrated at the center. "Density" in this sense is just mass divided by the volume of the space enclosed by the event horizon.